This invention relates to current generator circuitry. More particularly, this invention relates to current generator circuitry that can be selectively placed in a zero-current shutdown state.
The purpose of current generator circuitry or "bias" circuitry in an electronic circuit is twofold. It supplies the power necessary for portions of a circuit to operate and establishes the dynamic range in which the powered devices function.
Bias circuitry can be implemented in numerous forms. For example, a bias circuit suitable for use with a discrete NPN transistor may supply a positive voltage (V.sub.cc) to the NPN transistor's collector through a collector resistor (R.sub.c). The emitter of this NPN transistor may be connected to ground. In such an arrangement, the maximum amount of bias current (I.sub.c) that can be supplied to the NPN transistor can be determined by dividing the supplied voltage by the value of the collector resistor (i.e., I.sub.c =V.sub.cc /R.sub.c) . The amount of bias current actually drawn by a transistor, however, is usually dependent upon the magnitude of a drive signal supplied to its base. When the base drive signal is at its minimum, then so is the bias current, and vice versa. Thus, the NPN transistor's dynamic operating range is determined by the minimum and maximum amounts of bias current that can be drawn through the transistor's collector from V.sub.cc (again, which is dependent on the of drive signal provided to the transistor's base). For example, when an input signal less than approximately 600 mV is applied to the NPN transistor's base, substantially no bias current is drawn into the transistor's collector, at which point the transistor is in cutoff (the minimum point of the dynamic range). On the other hand, when a large enough signal is applied to the transistor's base, the maximum amount of bias current (I.sub.c) is drawn into the transistor's collector, at which point the transistor is in saturation (the maximum point of the dynamic range).
Fluctuations in the amount of bias current can significantly alter this dynamic operating range and adversely affect circuit operation. For example, circuit designers often select the operating point (Q-point) of a transistor using a load-line analysis technique that requires a constant DC bias current as an initial condition. Any significant change in that DC bias current alters the slope of the load-line and shifts the position of the Q-point. Such changes can cause transistors in a given circuit to go into cutoff or saturation at undesirable times and thus degrade circuit performance. It is therefore important that bias circuitry has the ability to provide a substantially constant amount of current, even if supply voltages vary.
Another important characteristic of bias circuitry is its quiescent current (i.e., the minimum operating current required by the bias circuitry when substantially no bias current is provided). It is generally desirable to reduce the quiescent current to the lowest possible value. One reason for this is the increasing demand for battery powered devices that have long "active" periods. Because the active periods of such devices are directly dependent on battery power, it is desirable to make this battery power last as long as possible. One way to do this is to reduce the amount of quiescent current used by bias circuitry in a given device.
An example of a prior art circuit is shown in Dobkin et al. U.S. Pat. No. 5,274,323 (the '323 patent). FIG. 1 is a schematic representation of the relevant portions of the current generator circuitry shown in the '323 patent (designated herein as current generator circuit 100). Current generator circuit 100 generally comprises a start-up section 101, a current supply (sourcing circuit) 102, and a bias section 103.
The purpose of start-up section 101 is to turn ON PNP transistors 120A-120E when a voltage differential first appears across the DRIVE and GND terminals. The start-up section includes transistors 110, 111, 112. Transistor 110 is a JFET produced by epitaxial growth and serves the purpose of providing current to diodeconnected transistor 111 when a voltage differential appears across the DRIVE and GND terminals. Transistor 111 is fabricated to have a high turn-on voltage (VBE approximately 850 mV at 25.degree. C.). With current flowing through transistor 110, transistors 111 and 112 turn On, sending current through resistors 121 and 122 and simultaneously drawing current from the common base node of transistors 120A-120E. This causes transistors 120A-120E, all of which have their base-emitter circuits connected in parallel, to turn on. The turning On of transistor 120E causes additional current flow through resistors 121 and 122. This additional current increases the voltage at the emitter of transistor 112 (i.e., across resistors 121 and 122) so as to eventually reverse bias the base-emitter junction of 112 and therefore shutoff start-up circuit 101 from the rest of the circuit after transistors 120A-120E have been turned on. Once transistors 120A-120E are operating, the components in start-up circuit 101 are of no consequence.
Moving further to the right of FIG. 1, NPN transistors 130, 131, and 132 form bias section 102. These transistors bias PNP transistors 120A-120E to provide a substantially constant current from all their collectors even with changing DRIVE voltage. This substantially constant current is also used to generate a substantially constant reference voltage across resistors 121 and 122. Bias section 102 can operate down to approximately one volt.
Transistors 130 and 131, which are connected in a current mirror configuration, have unequal emitter areas in a ratio of 10:1, causing a voltage of approximately 60 mV to appear across resistor 134 when transistors 130 and 131 conduct equal currents. The collector of NPN transistor 132 is connected back to the bases of transistors 120A-120E to provide a feedback loop. This feedback loop ensures that sourcing circuit 102 provides a substantially constant current even with changing voltage at the DRIVE terminal. Capacitor 133 is provided as frequency compensation for the feedback loop. Current generator circuit 100 turns off when the voltage supplied to bias section 102 drops below approximately one volt.
As can be seen from the above discussion, the current generator circuitry of the '323 patent will always be on and thus constantly draw substantial amounts of quiescent current whenever a sufficient DRIVE voltage is present to provide bias section 103 with approximately one volt of potential.
It would therefore be desirable to provide a current generator circuit that can be selectively turned off and placed in a substantially zero-current shutdown state independent of the DRIVE voltage so that quiescent current consumption is reduced.